Given a plot seed, how do you turn that into a scenario complete with clues, conflicts, NPC's, places, etc.?
e.g., players are Delta Green-style or Hunter the Vigil anti-supernatural secret agents, and the plot seed is that weird nearly abandoned town in PA that's been burning for decades (real story - it's on top of some mine). (just an example, not expecting you to finish this one for me :-)
Ask yourself a lot of questions:
I begin by asking questions. The 5 Ws work for starters. An example set that might spring to mind from the seed you mentioned are:
Don't Stop
I don't stop with a list like this, but this gets momentum going in thinking about the physical, social, and emotional environment between the seed and the players.
Once I have got this churning in my mind, I can start to think about what choices the characters might make if they begin an investigation, and the people that investigation would lead them to. That can lead me to an understanding of what locations and situations I may need to prepare as I consider where those people are, what they typically do, and how they might react to the characters when they meet.
Ideas are usually enough, but depending on how in sync you are with your players you may find more or less preparation to be useful in terms of locations and contacts.
All flesh must be eaten
Once I have all this fleshed out, I start thinking more deeply about how events happened, who is most likely to have been involved in making them happen - and based on who they are - how they could have done it. That leads me to a better sense of the clues that may have been left behind. Were they in a hurry? Do they know all they need to know to cover their tracks? Is everyone involved in the plan committed to making it work?
Let sit uncovered, then serve
I let this stew for a while (if I can) then review things from the top for the desired amount of coherence/confusion, the desired amount of freedom to act, and the desired amount of tension/time pressure.
The last thing then is to add players~
If you can put together a setting and actors with motivation, you can quickly construct a scenario.
It sounds like all you have at this point is a setting. Add some actors (individuals or organizations) with motivations, and you can build out from there.
Setting: What makes it unique, what obstacles and advantages does it present to player characters and to other actors in the scenario? How did the setting come to its present state? How long will it remain in that state if left uninterrupted?
Actors & Motivation: Who is involved, and what interest do they have in the setting? Are they of the setting or apart from it, hoping to influence it or take advantage of it? Do they know everything about the setting, or are some facts hidden to them? Do they want to preserve the status quo or alter it? What will happen if they are allowed to continue with their plans unchecked? You'll probably want to create two or three actors and think about their interaction. What would happen if you left them alone and the PCs never arrived?
Secrets (and clues to those secrets) are easier to plan if you know how motivation and setting interact. The Headless Ones want to keep the town burning so they can make it the center of an unholy ritual that will bring less than savory creatures into existence. What they don't know is that the town burns not because of some accident, but because Doctor Favius orchestrated the whole thing to get back at the people of the town who cast him out years ago for his demented rantings. The PCs are brought in by someone who suspects Favius of malfeasance, but when they catch him his mad rantings reveal that another, more dangerous party is involved.
Decide whether the scenario the party will face are static or dynamic.
An example of the first would be a dungeon crawl - or a sinister organization with no current agenda to do something bad (using the original analogy, Delta Green's Black Cod Island exemplifies this well). For whatever reason, the party is in the thick of it and has to deal with what they've found. A static encounter might have some sort of elaborate response algorithm, but whatever the consequences, they only comes into play once the party pokes the beehive with a stick.
A dynamic encounter involves someone who is trying to change the status quo, usually in a way the party won't want to happen. This is a little trickier, since you'll probably need a fleshed-out NPC villain, and some consideration about what happens once the players start meddling - along with what horrible things could ensue if they fail.
Somewhere in the middle - where I think you might be - is a mystery, where the players encounter the first example and have some reason to figure out why things are the way they are. Most often, an NPC from the second example was involved, and the players are trying to retrace his steps.
And finally, gaming nirvana is where you've finished the static setting, and the party is the dynamic force trying to change things.
